Various optoelectronic devices, such as photovoltaic cells and light-emitting devices, require the use of hole/electron-transport layers (HTL/ETL) for proper device performance. In these devices, most of the hole-transporting materials (HTMs) must be chemically p-doped to enhance transport efficiency before device incorporation. Examples of commonly used HTMs include chemically-doped poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), polyaniline (PANT), polypyrrole (PPy). However, these conductive HTMs are mainly only soluble in water and are often corrosive in nature, thus preventing their use in stable and moisture-sensitive devices.
Perovskite solar cells have experienced tremendous growth over recent years due to the low production costs of perovskite materials and power conversion efficiencies (PCE) approaching that of crystalline Si solar cells. The light-harvesting active layer in these solar cells is composed of a perovskite structured compound; most commonly, a hybrid organic-inorganic lead or tin halide ABX3, where A is an organic cation (typically CH3NH3+ or HN═CHNH3+), B is Sn2+ or Pb2+, and X is a halide anion that bonds to both. To date, methylammonium lead trihalide (CH3NH3PbX3, where X is a halogen ion such as I−, Br−, or Cr−), with an optical bandgap between 2.3 eV and 1.6 eV depending on halide content, represents the most efficient perovskite material. For example, CH3NH3PbI3 and CH3NH3PbI3-xClx have optical absorption coefficients of ˜105 cm−1 and yield PCEs over 15%.
Three major device architectures have been investigated with perovskite solar cells: (i) meso-structured perovskite solar cells (MSSC), (ii) positive planar heterojunction perovskite solar cells (positive PHJPSC), and (iii) inverted PHJPSC. In MSSC devices, the perovskite absorber is infiltrated within a mesoporous metal oxide scaffold (which can be an electron-transporting oxide such as TiO2, ZnO, or an insulating layer such as Al2O3, ZrO2) formed on top of a compact TiO2 blocking layer, which itself is deposited on a transparent conducting oxide substrate that functions as the anode. An HTM is often deposited over the perovskite to improve conductivity. A high work-function metal electrode is used as the cathode. PHJPSC devices have simpler device fabrication process and less production variation because no porous metal-oxide structure is needed and in turn, no pore-filling deviation with the perovskite. In these planar devices, the perovskite functions as the ambipolar layer in a p-i-n junction, where the intrinsic (i) layer is the perovskite absorber. In the positive PHJPSC configuration, an n-type conductor (e.g., TiO2, ZnO) is coated on a transparent conductive oxide substrate as the ETL, followed by deposition of the perovskite layer (PL), a p-type conductor as the HTL, and a high work-function metal as the cathode. In the inverted PHJPSC configuration, the sequence of the ETL, PL, and HTL is reversed; that is, a p-type conductor was coated on the transparent electrode, and an n-type conductor was deposited on the perovskite layer instead.
Despite rapid advances in the device architecture and fabrication technique of perovskite solar cells, the development of HTMs for perovskite solar cells has been limited. Specifically, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD or spiro), which has a twisted spirobifluorene core and a general bulky, three-dimensional structure, remains the HTM of choice and has reported the best device performance regardless of device architectures. However, the prospect of commercializing perovskite solar cells with spiro-OMeTAD as the HTM is poor for the following reasons. First, the current synthetic route to spiro-OMeTAD is tedious and expensive. Second, to improve its low intrinsic conductivity and mobility, spiro-OMeTAD needs to be oxidized, which is commonly achieved by introducing chemical dopants or additives (examples of which include Li-bis(trifluoromethanesulfonyl)imide (Li-TFSI) and tert-butylpyridine (tBP)) and exposure to air. Many of these dopants and additives are deliquescent, which is detrimental to device stability considering the moisture-sensitive perovskite absorber. Also, precise control over the oxidation process can be difficult, which greatly compromises device reproducibility. Further, the oxidation-promoted efficiencies often are short-lived, which casts doubts on their practical applications outside of the laboratory.
Accordingly, there is a need in the art for alternative hole-transporting materials for use in perovskite solar cells that can be produced via simpler synthetic routes and lower production cost, have compatible HOMO energy level relative to perovskites and high charge-carrier mobility, and most importantly, can provide high power conversion efficiencies without the use of chemical dopants.